1. Field of Invention
The present invention is related to a method for using a flash to assist in focal length detection, and more particularly to a method for looking up a characteristic exposure value table to assist in focal length detection.
2. Related Art
Nowadays, the auto focus of common digital cameras may substantially be divided into two types: one is active auto focus, and the other is passive auto focus.
In the active auto focus, a set of infrared transmitter or laser transmitter and corresponding receiver are used to project a pattern of light on an object to be photographed, and then a distance between the camera and the object to be photographed is calculated as a focal length through a method such as triangulation. Since the optical transmitter and receiver need to be additionally set in the camera when the active auto focus is used, the problem of high cost occurs.
In the passive auto focus, before the actual photographing, a camera lens of the digital camera is moved to a plurality of focus positions in the range from a focus position closest to the digital camera to infinity. The digital camera captures an image at the focus position and analyzes a definition to determine a focal length. The passive auto focus only needs to use an image sensor and an operation unit of the camera, and thus the manufacturing cost can be reduced.
However, since the passive auto focus has to rely on the definition calculated from the pre-captured image, when the object to be photographed is in an environment with insufficient brightness, the estimated focal length may have an error easily. In the photographing, if the environment has insufficient brightness, the camera cannot find the correct focal length easily because the definitions of the images are generally low when the definitions are estimated. Thus, the passive auto focus has the problem of less accurate focusing when the environment is not bright enough.